Group calls, instant messaging, interactive gaming and Push-to talk over Cellular (PoC) are examples of optional communications services that may be provided by a communications system, and which involve the participation of a plurality of users of the communications system. Subscriptions by users of the communications system to such optional communications services are conventionally achieved only by separate individual requests from the users through a customer care facility or a self-service portal. The inventors have identified the problem that one user wishing to participate in such an optional communications service with another user depends on such another user already having made its own individual request for subscription to the service, and that this can lead to the service not being used as effectively as possible.